Piezoelectric transducers may be used in a range of applications, including imaging tissues of a human body by electrically exciting an ultrasonic transducer to generate short ultrasonic pulses that are caused to travel into the body. Echoes from the tissues are received by the transducer and are converted into electrical signals. The electrical signals are then amplified and used to form the medical image of the tissues or the anatomy under examination.
One concern in the design and operation of an ultrasonic transducer is minimizing signal ringdown. At the termination of emission of a single acoustic waveform, a radiating surface of the transducer, signal ringdown is manifested as a series of minor acoustic waves. Ringdown is a result of reverberations taking place within the piezoelectric transducer as wave energy reflects off the opposed surfaces of the structure. For example, wave energy that reaches the radiating surface is divided between escaping energy and reflected energy. The degree to which the energy is reflected depends upon the reflection coefficient, which depends on the acoustic impedance match between the piezoelectric element and the medium contacting the piezoelectric element. Conventionally, a matching layer is provided between the piezoelectric element and the load medium, e.g., tissue or water.
Signal ringdown has a number of adverse effects on the performance of the transducer and consequently the imaging system. Perhaps most importantly, reverberations reduce the bandwidth of the device, with a corresponding increase in pulse duration, i.e., ringdown. An increase or decrease in the pulse duration decreases or increases the spatial resolution of a transducer used in an imaging application. It also follows that enhancing the bandwidth will improve the penetration depth into the load medium and the ability to more efficiently receive echoes from greater depths.
Techniques for reducing reverberations within a piezoelectric transducer are known. As previously noted, an acoustic matching layer may be formed at the radiating surface of the piezoelectric material. The matching layer typically has an acoustic impedance between those of the piezoelectric material and the load medium, thereby acting as an intermediate in the transition of impedance to acoustic waves from the piezoelectric material. However, this requires the availability of a suitable material, as well as suitable processing. Another technique is to attach a backing layer at the back surface of the piezoelectric material. The backing layer may be selected to match the impedance of the piezoelectric material and to absorb any wave energy that has been transmitted rearwardly, at the expense of a reduction in sensitivity. While other techniques are known, further improvements in reducing ringdown time are possible, each with their own limitations and increased processing steps.
What is needed is a transducer device that has structure to reduce ringdown time, thereby enhancing performance.